1. Field of the Invention
The present invention relates to a polymerization fluid with a small environmental burden to be used for electropolymerization of a 3,4-disubstituted thiophene (hereinafter referred to as “substituted thiophene”), and a production method thereof. The present invention also relates to a conductive polymer film with high conductivity and transparency and a transparent electrode with the conductive polymer film, which are obtained from the polymerization fluid.
2. Description of the Related Art
Polymers of a substituted thiophene are conventionally known, and have been applied in various fields such as a solid electrolytic capacitor, a polymer battery, an antistatic film, an indicating element, a sensor, and an electrode material. Particularly, poly(3,4-ethylenedioxythiophene) (hereinafter, 3,4-ethylenedioxythiophene is referred to as “EDOT” and poly(3,4-ethylenedioxythiophene) is referred to as “PEDOT”) is suitable for various electrical and optical purposes because it has a low energy band gap, very high conductivity and environmental stability, and excels in permeability in relation to visible light (transparency). These conductive polymers are obtained by chemical polymerization or electropolymerization (for example, see patent literature 1 (JP 1-313521 A)). When electropolymerization is implemented, a film-shaped conductive polymer with high conductivity is formed on an electrode from a small amount of substituted thiophene in a short period of time.
The electropolymerization of substituted thiophene is usually performed with a polymerization fluid containing an organic solvent such as acetonitrile and isopropyl alcohol, because the solubility of substituted thiophene in water is remarkably low. For example, in Examples 9-14, 18-22 of the patent literature 1, a polymerization fluid containing acetonitrile as a solvent and EDOT as a monomer is used to obtain a film with conductivity of approximately 200 S/cm. An organic solvent such as acetonitrile dissolves an oligomer produced in the process of polymerization to make it a high-molecular-weight polymer, and produces a conductive polymer film having high conductivity. However, there have been cases where the conductive polymer film produced was dissolved in the organic solvent, the density of the film was lowered, the smoothness of the film surface was lost, and the transparency of the film was decreased. Further, the use of an organic solvent generally increases an environmental burden and is economically disadvantageous compared with the use of a water solvent. Furthermore, many organic solvents are harmful to the human body, and in the case of electropolymerization using a combustible solvent, preventive measures need to be taken to avert the risk of fire due to electric sparks.
Therefore, consideration has been given to the use of water as a solvent of a polymerization fluid, which is inactive against the conductive polymer film produced, has a small environmental burden, and is economically efficient. The following two methods have been proposed so far—a method to add an anionic surfactant with a long-chain alkyl group such as sodium dodecyl sulfate or sodium dodecylbenzenesulfonate or a polymerization-type anionic surfactant such as sodium polystyrene sulfonate to a mixture of substituted thiophene and water so that the substituted thiophene is solubilized or emulsified, and a method to irradiate ultrasonic waves through a mixture of substituted thiophene and water and disperse oil drops of substituted thiophene into water.
For example, non-patent literature 1 (Langmuir, 1999, 15(7), 2566-2574) describes electropolymerization using a water solution in which EDOT, LiClO4 as a supporting electrolyte, and sodium dodecyl sulfate as an anionic surfactant are dissolved. By the action of sodium dodecyl sulfate, the solubility of EDOT in water is increased, the oxidation potential of EDOT is lowered, and a film with an excellent adherence property and a regularly-arranged structure is produced on a Pt electrode. Patent literature 2 (JP 2000-269087 A) describes electropolymerization using a polymerization fluid in the form of aqueous medium produced by emulsifying a thiophene derivative such as EDOT with an alkylnaphthalenesulfonate surfactant. The alkylnaphthalenesulfonate anion taken into the polymer layer as a dopant is so bulky that de-doping is inhibited, and a conductive polymer layer that is stable at high temperature and high humidity is obtained.
Non-patent literature 2 (Synthetic Met., 2009, 159(5-6), 406-414) describes electropolymerization using an aqueous solution in which EDOT and sodium polystyrene sulfonate as an anionic surfactant are dissolved. By the action of sodium polystyrene sulfonate, the solubility of EDOT in water is increased, EDOT becomes more easily oxidizable, and a uniform film is formed on a Pt electrode. Patent literature 3 (JP 2006-295184 A) describes a method for producing a conductive polymer layer using a dispersion prepared by adding EDOT, iron (III) sulfate, and sodium persulfate to an aqueous solution containing polystyrene sulfonic acid, forming PEDOT/polystyrene sulfonate particles in the aqueous solution by the reaction between them, and adding a binding agent to it.
Furthermore, non-patent literature 3 (J. AM. CHEM. SOC. (2005), 127(38), 13160-13161) describes results of electropolymerization using an opaque polymerization fluid in which narrowly-distributed EDOT oil drops with an average diameter of 211 nm are dispersed in water. The polymerization fluid is prepared by adding an amount of EDOT larger than saturated solubility to an aqueous solution in which LiClO4 as a supporting electrolyte is dissolved, and irradiating ultrasonic waves at a frequency of 20 kHz with power of 22.6 W/cm2 for 60 seconds (see FIG. 1 in this literature). By direct charge transfer between a Pt electrode and the EDOT oil drops, a PEDOT film having conductivity of approximately 30 S/cm is formed on the PT electrode.